. Chordate anatomy. Chordata; Anatomy, Comparative. THE SKELETAL SYSTEM i8i In mammals, the coracoid is reduced to a process fused to the scapula. In man, in addition to the coracoid process, a remnant of the coracoid bone survives in the coracoid ligament which extends from the coracoid process to the sternum, and in which occasional pieces of cartilage are found as rudiments of the coracoid. The clavicle has supplanted the precoracoid, remnants of which, however, usually occur within the clavicle. See Fig. 170. The mammalian hip bone differs Httle from that of reptiles. The number of sacral
. Chordate anatomy. Chordata; Anatomy, Comparative. THE SKELETAL SYSTEM i8i In mammals, the coracoid is reduced to a process fused to the scapula. In man, in addition to the coracoid process, a remnant of the coracoid bone survives in the coracoid ligament which extends from the coracoid process to the sternum, and in which occasional pieces of cartilage are found as rudiments of the coracoid. The clavicle has supplanted the precoracoid, remnants of which, however, usually occur within the clavicle. See Fig. 170. The mammalian hip bone differs Httle from that of reptiles. The number of sacral vertebrae to which the coxal bone is attached increases in mammals. In man there are five sacral vertebrae, to three of which the hip bone is attached. Evolution of the Free Extremities. Two contrasting types of free extremity appear in vertebrates, the fins characteristic of fishes and the VLRTEBRAL MARGIt. 5-STERNAL EXTREMITY Fig. 171.—Human pelvic and pectoral girdles in lateral aspect. A is the pelvic girdle of the right side and B-C the pectoral girdle of the same side. toed appendages such as are found in the remaining classes from amphibians to man. The conversion of the one into the other continues to be a vexed cjuestion of vertebrate morphology. Technically stated, the problem has been to determine how the evolution of the ichthyopterygium into the cheiropterygium has occurred. Interest has centered especially in the transformation of the skeleton. Primarily the fish fin, like that of the fossil shark Cladoselache, was supported by radial cartilages which articulated with basalia, of which one or more articulated with the girdle. In the pectoral fin of modern elasmobranchs three basalia, propterygium, mesopterygium, and meta- pterygium (Fig. 172^), connect the fin with the girdle. Morphologists, however, disagree as to the skeleton of the primitive extremity, the archipterygium. While some suppose it to have been uniserial, , the radial cartilages were limited t
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